In many applications involving optical signals, it is important to be able to discriminate between signals with different temporal structure even though they may contain similar amounts of energy. In particular, some optical communication systems, such as optical code-division multiple-access (OCDMA), rely on this ability. In OCDMA systems, such as described in co-pending U.S. patent application Nos. 09/100,592, filed Jun. 19, 1998, and 09/115,331, filed Jul. 14, 1998, multiple optical channels are multiplexed by impressing a specific temporal code onto the bits comprising a specific channel and then combining the multiple channels in a common transport mechanism wherein the multiple channels remain distinguishable at least on the basis of the impressed time code. Demultiplexing can be accomplished using filtering devices providing multiple output channels, each of which comprises an optical signal representative of that portion of the multiplexed data having a specific time code. The filtering devices may operate by various mechanisms. An exemplary mechanism uses the cross-correlation of the multiplexed data stream with a reference temporal waveform. Typically, but not necessarily, the reference waveform is one of those waveforms utilized in the OCDMA channel-encoding scheme. The temporal waveform of signals produced by the demultiplexer in a specific output channel subsequent to the arrival of a particular time code on the multiplexed transmission channel depends on the degree of similarity between the input time code and the output channel reference time code.
Specifically, close matches of input and reference time codes typically produce output signals including a temporally brief, high power subsignal. Occurrence of a brief, high power subsignal is then indicative of a close match. The overall energy of output signals displays sensitivity to a degree of match between the input and a reference time code, but the sensitivity may be weak. The degree of match between input and reference time codes can be ascertained by time-resolved detection of the output signal since such detection reveals the presence or absence of brief, high power subsignals. Apparatus for direct time-resolved detection are known in the art, but they require the use of expensive, high-speed detection equipment. The present invention provides a means of processing output channel signals so as to provide a robust means of differentiating between output signals of similar energy but different temporal waveform without the need of time-resolving the temporal waveform of the output signals.
In order to identify those optical signals in a specific output channel that contain high-power subsignals and to discriminate them from output signals containing similar energy but no high-power subsignal, nonlinear optical processes can be used according to the present invention. By high-power subsignal, we mean a temporally short pulse of optical energy. Such high-power subsignals are indicative in OCDMA applications of a close match between the time code of an input bit and the output channel's reference time code. The time-integrated output from a detection system incorporating at least one nonlinear element in conjunction with proper threshold detection can distinguish optical signals containing short, high power subsignals from those that do not, even though the detection lacks the ability to temporally resolve the high power subsignal.
The present invention relates specifically to the use of time-integrated nonlinear detection for the purpose of signaling the presence of high power subsignals. In general, any response that depends nonlinearly on the input optical intensity will serve the intended purpose.
A first common form of optical nonlinearity is second-harmonic generation (SHG). This type of interaction has been used for many years as a way of measuring the temporal waveform of short pulses of light as described by Naganuma et al., U.S. Pat. No. 4,792,230. The present invention does not relate to the measurement of temporal waveforms. The present invention relates to the determination of the presence of a high power subsignal within an optical signal without the need to temporally resolve the optical signal.
A second form of second-order effect that can be used for the purpose of the present invention is two-photon absorption (TPA). Specifically, two-photon-induced photocurrent in semiconductor devices is especially useful since the nonlinear material and the photodetector are integrated into the same physical device. In addition, unlike SHG, TPA is largely polarization-independent and does not require phase matching and is therefore simpler to implement. The main constraint on TPA optical detection is that the semiconductor bandgap must be larger than the photon energy (to minimize linear absorption) and smaller than twice the photon energy. Conventional semiconductor waveguides, photodiodes, LEDs, and laser diodes have all been demonstrated to produce TPA photocurrents (see, for example, Reid et al., "Commercial Semiconductor Devices for Two Photon Absorption Autocorrelation of Ultrashort Light Pulses," Optics and Photonics News, vol. 9, 1998).
In general, the signal S.sup.(n) from a time-integrated nonlinear detector has the following form: EQU S.sup.(n) =.eta..sup.(n) .intg.dt(I(t)).sup.n (1)
where I(t) is the instantaneous intensity versus time of the optical input to the detector, .eta..sup.(n) is the nonlinear coefficient, and n is the order of the nonlinear process. For linear detection (n=1), the time-integrated signal is simply proportional to the total energy of the input waveform, i.e., the integrated area of the intensity function properly normalized. For (n&gt;1), the output signal depends on the temporal form of I(t). This can be seen from the following simple examples shown in FIG. 1. A rectangular pulse that has a temporal extent of two units and an intensity of one unit has a time-integrated linear signal strength S.sup.(1) of two units. This is identical to the signal S.sup.(1) of a pulse with a temporal duration of one unit and an intensity of two units. However, the signal S.sup.(2) of the same two respective pulses has a ratio of 2:4. Therefore, while the two waveforms shown in FIG. 1 have the same total energy, they can be distinguished based on their time-integrated nonlinear signal strength--the shorter-duration waveform will have a larger time-integrated nonlinear signal.
The present invention comprises an apparatus for the detection of optical signals that uses nonlinear optical interactions and provides for the differentiation between optical signals of similar energy but differing temporal waveform without requiring explicit temporal waveform resolution. The apparatus includes a light diffracting means that is programmed according to a particular temporal function and a first means for causing input optical signals to be incident on the diffracting means. The apparatus also includes a second means for focusing the light emitted by said diffracting means to a particular point in space referred to as the focal point. The light emitted by the diffracting means possesses a temporal waveform representative of the cross-correlation of the input optical signal and the particular temporal function that is programmed into the diffracting means. A nonlinear optical device that produces an electronic signal in response to excitation by light is placed at the focal point of the focusing means. The electronic signal possesses an amplitude that varies nonlinearly in response to the energy content of the input light and varies slowly compared to the instantaneous input optical intensity. Alternatively, a nonlinear optical device that produces an optical signal in response to excitation by light can be placed at the focal point of the focusing means. The optical signal possesses an energy that varies nonlinearly in response to the energy content of the input. The nonlinear optical signal is then converted by a linear optical detector to an electronic signal whose amplitude varies slowly compared to the instantaneous input optical intensity. The present invention also includes an electronic thresholding device that generates an output electronic signal in response to those electronic signals produced by the nonlinear optical device which exceed a preset threshold.
Another aspect of the present invention includes an apparatus for normalizing the electronic signal generated by the nonlinear optical device. The normalizing apparatus includes a linear optical device that produces an electronic signal in response to the light emitted by the diffracting means. The linear optical device is positioned such that it intercepts a portion of the light emitted by the diffracting means. The electronic signal produced by the nonlinear optical device is divided electronically by the electronic signal produced by the linear optical device raised to an algebraic power. The linear optical device has a response that varies slowly in response to the incident optical intensity. The methods provided are especially useful in situations wherein optical signals representative of auto- and cross-correlations are to be distinguished on the basis of the intense, short temporal feature generally found in the auto-correlations but not found in cross-correlations. The correlations of interest can be generated by one of a variety of methods known in the art to produce optical signals representative of correlations between input optical signals or between input optical signals and preprogrammed reference signals. The present apparatus and method utilize detection devices and systems that individually or collectively prohibit the direct observation and manipulation of the temporal profiles of the optical signals to be distinguished.